During the production of semiconductor wafers for integrated circuits (ICs), optical scanning for defects or contaminants on a surface of the wafers is common. Such scanning can involve focusing a beam of light to a spot on the surface and detecting whether the light is scattered, as opposed to being specularly reflected from that spot, thereby signaling the presence of anomalies, i.e., particulate contaminants or pattern defects, at that spot. It is often useful to inspect wafer surfaces for anomalies at various stages during such production.
It is known that particle to background signal contrast can be optimized by directing the beam obliquely at the wafer surface. Each time a wafer is scanned, it is important that the surface is accurately positioned, in order that the scanning beam is focused on the surface and to ascertain the position on the surface of any particles or defects. This is especially true for detecting anomalies using periodic feature comparison where a slight change in height or tilt of the wafer surface can lead to meaningless results, because periodic feature comparison requires near perfect registration.
In U.S. Pat. No. 5,272,517, Tokura teaches a height measurement apparatus in which a laser light beam is directed normally toward a surface to reflect from an inclined feature, such as a solder flow, connecting an IC with a printed circuit board (PCB). Recognizing that the surface provided reflected light with varying intensity which altered the height measurements, Tokura vibrated a position of the laser light beam, relative to the surface. The light reflected from the surface is collected at a detector and temporally averaged into a mean height while the position of the laser light is vibrated. The mean height is determined to be the final height calculated.
U.S. Pat. No. 5,166,516 to Kajimura teaches a slant detection and control mechanism for a scanning probe microscope in which a light beam is reflected off a back surface of a cantilevered beam having a probe on a front surface. The reflected beam undergoes multiple reflections to arrive at a detector that determines the deflection of the probe toward an object being scanned.
In U.S. Pat. No. 4,748,333, Mizutani et al. teach a surface displacement sensor having multiple slits and lenses aligned along a trajectory of light reflecting from the surface at an oblique angle, and a sensor positioned at an end of that trajectory which receives increased light when the slits are aligned with the light. The multiple slits are provided to overcome inaccuracies in the height measurements due to differing thin film thicknesses which varies the intensity of the reflected light. At least one of the slits can be narrowed for increasing the accuracy of the surface displacement.
U.S. Pat. No. 4,864,123 to Mizutani et al. also utilizes the alignment of multiple slits and lenses along a trajectory of light that is obliquely reflected off a surface to determine the level of that surface. This patent teaches the advantage of polarization correcting optics for adjusting the intensity ratio between orthogonal polarized components of the reflected light.
It is an object of the present invention to devise an accurate measurement of surface height for a wafer inspection system without being adversely influenced by variations of intensity of reflected light.
It is another object of the present invention that the measurement of the surface height is accomplished without regard to the height variations due to patterned features on the surface.
It is another object of the present invention to measure surface height using the same source of light that is used for particle inspection of the system.
It is yet another object of the present system to provide means for using the measurement of the surface height to adjust a position of the surface during particle inspection.